So in this video we're going to discuss modifying BGPs Route Decisions and we're going to modify the Route Decisions using the Local Preference this time. In the last video, we showed you what happened if you used the Weight. You saw that Access 1 said hey I've got all the best routes to this network down here and Access 2 said well no, I've got all the best routes to these networks and so as a result, you had inconsistent routing, depending upon which Router you happened to hit, you got routed out a different Internet pie. That's not really the way to go so we're going to use the Local Preference and the Local Preference is exactly what it sounds like, it's, it's local to the AS and it is pretty much equivalent to the Weight. Basically saying, hey everybody else, I have the best routes to the network in question or all networks and so you need to send all of your traffic out through me. Really, the only difference between the Weight and the Local Preference is if the weight stays on the Access 1 Router, the Local Preference gets advertised out to the Access 2 Router so that Access 2 also believes hey, he's got the best routes for whatever network in question and so I'm going to send my traffic over to him. Now the only thing you really have to know for the exam, because they're not going to get real deep into modifying the Local Preference, the only thing you'd have to know for the exam is to go into Access 1 for example, go to Config T, Router BGP and the AS number, in this case, AS 12 and you'll enter the Command, BGP Default Local Preference and set it to a number between 1 and 4.2 billion. Any number other than 100 or any number higher than 100 will effect the Routing Decisions and say hey, I have all the best routes. Conversely if you wanted to say, hey I have the worst routes, don't ever send any of your traffic through me unless you have no other way out of the network, you could set the Local Preference to less than 100 and that means that any other Router would automatically win because you're advertising your routes, saying hey my routes are inferior to yours, I don't care where your routes go, mine are inferior because maybe this T1 connection is metered. You're charged by the amount of bandwidth you use or the number of megabytes you send across it. Or this T1 connection quote unquote is actually a satellite connection that has a really, really high latency. So you can use it but you only want to use it as a last resort and that's the situation where you'd want to set a Default Preference that's lower than 100. In this case, keeping with our example, we'll set the Default Preference to 150 so that sets the Default Local Preference for any routes that come into this Router from anywhere to 150. So that means this guy will always win unless you got another Router out there that's advertising a Local Preference of 200 or 300 0r 17 million or whatever number you want to put in there. If you make a change like this to the Default Local Preference you have to clear the BGP Table by doing Clear IP BGP star. Once you do that, it will tear down the Neighbor Relationships and it will bring them back up. Now there is a command you can use which is Clear IP BGP and if you choose, let's hit Star, if you choose Soft, that is a Soft Reconfiguration and will not tear down all the Neighbor adjacencies but it will start a Route Update back and forth between them and that's something you, you'd use if you had a 24 by 7 shop and you couldn't afford to tear down the neighbors for this particular router. You could do a Soft Reconfig. I still wouldn't recommend it, because you still will 00:03:24 ] impact routing because you're sending Route Updates all over so you could potentially use up a lot of Bandwidth. But if you absolutely positively had to reset the Neighbor adjacencies and couldn't pull them down to do so, this is the way to do it. So now I've blabbered on enough and the Neighbor adjacencies have come back up. So if do Show IP BGP on the Access 1 Router, you'll notice that he doesn't see any Local Preferences other than 100 because he's not going to show you the Local Preferences that he's advertising because obviously it doesn't matter to him what his Local Preference is. However, if we look over on Access 2, Show, Show IP BGP and you'll notice that any routes that come from 10.20.1.2 have a preference of 150 and you'll only see 150s on the Access 2 Router because again, it knows that it's Local Preference is 100, so it doesn't need to show what it's Local Preference is. As a result, all of the routes in the network are now going out this Access 2 Router. You'll notice that, that is the one that wins the best route for each one of these networks. And if we do Show IP Route, the Routing Table on the Access 2 Router reflects that. It shows all the BGP routes going out 10.20.1.2, none of them going out the local T1 to 188.8.131.52 which is the Loopback Peer on the other end of this dual T1 link here. And obviously as I mentioned it is local to the AS. If we go over to ISP 1, we do Show IP BGP on this guy, you'll notice that he's not getting any Local Preferences from 184.108.40.206, because again it's Local Preference and 220.127.116.11 is outside his Autonomous System. It's an AS 12, so again since it's local it doesn't care what it is. Now that's the way you'd do it for the exam. There's a better way and, and I think it's a cooler way because it involves Route Maps and it specifically targets just this one network down here, this 18.104.22.168 network down here, modifies the Local Preference for that guy and then advertises that elevated Local Preference out to the rest of the network, so that just this one network goes through the Access 1 Router. I'm looking at the time here and I see that I'm not going to have the time to do it in this video, so what we'll do, is we'll go back into Access 1, we will clear that Default Local Preference. Do Config T, Router BGP 12, No Default and just to hit Question Mark here, oh, I got to put BGP in front of there. No BGP Default and these are all the things you can set Defaults for. All three of them. You can change the Route Target or the Local Preference or you can change the Unicast Mode. Obviously we're going to do No BGP Default Local Preference. And we'll do Clear IP BGP star and we will wait for the Neighbor adjacencies to come back up and we'll pick this up in the next video.
|Course:||Implementing Cisco IP Routing (642-902 ROUTE)|
|Duration:||10 hrs / 105 lessons|
|Captions:||Available on CD and Online University|
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